The regeneration of hydrochloric acid pickling waste, which involves the recovery of hydrochloric acid from waste left after a pickling operation on various steel products, has led to three primary areas of concern regarding the operation of the recovery system. First, hydrochloric acid availability has been materially reduced with a corresponding increase in cost throughout the industry. Second, the heat or energy necessary to operate a recovery process is less available and at an increased cost. Third, the environmental disposal of waste hydrochloric acid pickle liquor is becoming a critical factor in all systems designs.
The prior art method for treating hydrochloric acid pickle waste as illustrated in U.S. Pat. No. 3,635,664 to Morimoto issued Jan. 18, 1972 involves an addition of sulphuric acid to the hydrochloric acid pickling waste to convert FeCl.sub.2 in the waste to HCl and FeSO.sub.4. Then the resulting mixture is distilled to vaporize substantially all of the HCl together with the water and to precipitate the ferrous sulphate. Finally, the HCl and water are condensed to recover hydrochloric acid while separating out the precipitated ferrous sulphate from the residual liquid and circulating the resultant liquid as a sulphuric acid source. The Morimoto patent uses flash distillation to vaporize the hydrochloric acid in the mixture of waste hydrochloric acid and sulphuric acid.
Some of the difficulties inherent in the Morimoto process include a need to concentrate the sulphuric acid in the mother liquor before it can be recirculated back to the reactor which, of course, requires an additional amount of energy to remove the water from the sulphuric acid, especially at the high reactor temperatures. That is, the heat of vaporization of water from sulphuric acid is high compared to that from ferrous chloride solution.
The patents to Morgan No. (1,626,623), Whetzel (2,005,120), Smith (2,118,272), Edge (2,395,729), Swindin (2,616,790), Shaw (2,662,812), Martin (2,668,130), Francis (2,880,062), Sweet et al (2,960,391), Umstead (3,713,786) and Morimoto (3,743,484), are addressed to processes for recovery of metals from pickle waste which does not contain hydrochloric acid. The patent to Senior et al (3,787,306) is addressed to a metal recovery system using oxidation and neutralization which requires substantial energy expenditure. The patents to Minkoff et al (1,798,099), Adamson (1,764,593) and Haen (687,994), show common hydrochloric acid production methods unrelated to pickle waste recovery.
The hydrochloric acid recovery method of the present invention overcomes the difficulties of the prior art in supplying sufficient energy to remove water from the waste solution in order to complete the waste removal process.